Perhaps some of the reasons are here.
https://www.healthline.com/health-news/can-a-sudden-drop-in-temperature-make-you-sick
According to health experts, however, it’s not cold weather itself that makes us sick — but lower temperatures do increase our risk for infections in a few ways.
“Data indicates that viruses survive and proliferate more effectively at colder temperatures, allowing them to spread and infect greater numbers of people in colder temperatures,” said Dr. Robert Glatter, an emergency physician with Lenox Hill Hospital in New York City.
“Coupled with this, colder weather can blunt the immune response, increasing the chance that you may get sick,” he said.
Edit...
Jumping down the page we find....
In addition, colder temperatures provide the flu virus with a protective layer, making it firmer and less penetrable, Glatter explains.